1. Field of the Invention
The present invention generally relates to a hydraulic controller for an automatic transmission which hydraulically controls the speed change mechanism of the automatic transmission. More particularly, the present invention relates to a hydraulic controller for an automatic transmissions of a vehicle.
2. Description of Related Art
Conventionally, an automatic transmission (AT) which is widely used in vehicles to smoothly transmit torque according to varying loads has achieved speed change control using a hydraulic circuit having a plurality of dispersed hydraulic control valves, accumulators and solenoid valves mounted therein. Therefore, such conventional ATs are large in their size and number of parts, complicated in their structure and high in their cost. In order to solve these problems, an idea has been proposed, as disclosed in Japanese Examined Patent Publication (Koukoku) No. 4-77183, that speed change is controlled by one integrated control valve. Implementation of this idea can downsize, lighten and simplify the hydraulic controller used in the conventional AT.
If the oil pressure is to be directly varied to control the oil pressure of each friction engaging element, the AT requires one hydraulic control valve for each friction engaging element, and the hydraulic controller thus becomes larger and less economical. For this reason, the integrated valve is used for hydraulic control. In order to achieve the proper oil pressure distribution according to the engagement state of required friction engaging elements, automatic speed change is achieved by smoothly switching the oil pressure with as few hydraulic control valves as possible by using the hydraulic valves of the integrated valve and by further switching the hydraulic valves with a hydraulic controller.
In a system in which dispersed hydraulic control valves control each friction engaging element for speed change control, speed change control is achieved by specifying a speed change instruction (which selects friction engaging elements within the AT to obtain the proper gear ratio) by operating a gearshift (with which a driver can select a desirable gear position freely to some extent) and an engine control computer (i.e., an electronic control unit or ECU) based on an opening of an engine throttle and vehicle speed. In this case, even if the automatic control function of the ECU fails, the driver can select forward and reverse gears and forward speed gear changes to some degree by operating the gearshift. However, in the above conventional system which controls the automatic speed change using the integrated valve, since the valve switching is controlled only by the ECU, in the event of a failure in the ECU itself or of the occurrence of an error such as a wiring disconnection, the speed change function may be completely lost.